The present invention relates to a hybrid circuit in the interface circuit between a subscriber circuit and a telephone switching system in a telephone exchange system and, in particular, relates to such a circuit which is suitable for fabrication in an integrated circuit.
First, a prior hybrid circuit is described in accordance with FIG. 1. In the figure, the reference numeral 1 is a telephone set or a subscriber terminal, 2 is a subscriber line, 3 and 4 are input and/or output terminals of a subscriber interface circuit, 5 is a power source, 6 is an inductance in a speech current supply circuit, 7 is a hybrid circuit which has a hybrid transformer 8 and the resistors 9 and 10 and the circuit 11 having a resistor and a capacitor. The reference numerals 12 and 13 are terminals coupled with a modem (not shown) for transmitting and/or receiving a speech signal, 14 is a relay contact for detecting the ON or OFF status of the loop circuit in the subscriber terminal 1, 15 is an output terminal of said relay contact 14, and 16 is a capacitor for preventing DC current.
In the above circuit, a speech current is supplied from the DC power source 5 to the subscriber terminal 1 through the inductances 6, each of which has usually the DC resistance 220 ohms. The connection or the disconnection of the loop circuit of a subscriber terminal 1 is detected by a relay contact 14 and the output terminal 15. The speech signal from the subscriber terminal 1 is terminated by the resistors 9 and 10 each having the resistance r.sub.0 through the hybrid transformer 8, and said speech signal is also applied to the modem (not shown) through the terminals 12. It is assumed that the impedance of the capacitor 16 is small enough as compared with the impedance of the inductances 6 and the impedance of the hybrid transformer 8. Assuming that the winding ratio of the hybrid transformer 8 is L:M, then, the resistance r.sub.0 of the resistors 9 and 10 has the equivalent effect of 2(M/L).sup.2 r.sub.0 at the primary side of the hybrid transformer 8. On the other hand, the speech signal from the modem (not shown) through the terminal 13 is transferred to the subscriber terminal 1 through the subscriber line 2. It is assumed that the circuit 11 has the impedance M.sup.2 Z.sub.L in order to minimize the stray signal from the input terminal 13 to the output terminal 12 through the hybrid circuit, where Z.sub.L is the impedance of the subscriber line 2 and the subscriber terminal 1 at the terminals 3 and 4.
However, the prior hybrid circuit of FIG. 1 has the disadvantage that the size of the circuit can not be small, and the weight of the circuit can not be light, because the transformer 8 must be large since both the AC current and the DC current flow in the transformer 8 and thus the DC resistance of the coils of the transformer 8 must be small, and the capacitor 16 must have large capacitance for improving the transmission characteristic for the speech signal. Further, the characteristics of the hybrid transformer 8 is not stable, because the trasmission characteristics of the transformer 8 depends upon the DC current superposed on a speech signal.